A printing apparatus is generally incorporated in a rear section of a word processor or is connected to a personal computer through a cable thereof. In recent years, various types of printing apparatus have been introduced into the market, and the products are mostly based on the dot matrix system in which an image is formed by printing many small and thin dots on a printing medium. The thinner and smaller the dots are, the more beautifully characters and drawings can be printed. There are many printing systems including, but not limited to, the thermal head system in which thermal dots are formed by a heat-generating resistor, laser printer system in which dots are formed with a laser beam, and ink-jet printer system in which fine particles of ink are blown to a surface of a form.
FIG. 10 is a block diagram showing one example of the conventional type of printing apparatus. This printing apparatus 500 is a general dot matrix printer. Print data transmitted from a computer C is once stored in a buffer 501 incorporated in the printing apparatus 500. When the buffer 501 is full, a control section 505 of the printing apparatus 500 sends a control signal to the computer C, and data transmission is once stopped. When print data is character data (ASCII code), character data having a corresponding dot pattern is selected from a bit map table in a font 502 and transmitted to the control section 505. The control section 505 drives a print head 503 according to received print data for an image or a character. The print head 503 has a structure in which a plurality of pins are arrayed in a row and the pins are moved toward paper by magnetism. When the moved pins contact the an ink ribbon, ink is placed on a form in the contrary side and an image is formed there.
FIG. 11 is a reference view for illustrating a data transfer system of the printing apparatus 100. The image shown in the figure comprises a group of thin dots g. When printing an image, print data for each unit dot constituting the image is sent from the computer C to the printing apparatus 500. Also when printing a character, print data is sent dot by dot according to a bit map font. A dot position is identified by XY addresses on a form. A physical limit of resolution of a print image is a size of the dot g as the minimum unit, but the resolution can be made higher by changing the way of printing the dots.
With the printing apparatus 500 based on the conventional technology, however, all print data is sent dot by dot, so that a data rate is generally large. For this reason, a long time is required for data transfer, which is disadvantageous. Also the buffer 501 easily enters the busy state, which in turn results in deterioration of efficiency in a printing work.
Further, resolution of a print image is uniform, and the resolution can not be changed freely. For this reason, when an image not requiring high resolution is printed, a volume of transmitted print data becomes excessive, and on the other hand, when an image is printed in a zoomed-up form and high resolution is required, the resolution can not be made higher than a certain value, and a satisfactory image can not be obtained for printing, which is disadvantageous.